Gregory is being recruited as an athlete, if he could beat out the other qb's he would play there, but it seems like they would expect him to be a WR, which we need.
Alipate has never shown the ability to play those other positions, if he can great, but he has not demonstrated that. Parrish is a good athlete, and he is pretty tough(he is thin and I don't think he could take the poiunding in this offense), but moving to another postion would be tough as well. It will be interesting to see what he does.